Measurements of characteristics of transistor power switch devices are performed at various stages of their production and use. For example, the supplier makes non-destructive measurements of operating parameters and physical characteristics of wafers and dies to detect risks of defects in production batches and to control variation of production process parameters and makes potentially destructive tests of die samples, measuring the effects of the tests on the samples. Any failure of delivered products in the field during use usually results in the customer returning the failed products to the supplier for analysis, which again leads to measurement of the characteristics of the failed product.
Examples of measurements which may be made include semiconductor horizontal and vertical doping profiles and dimensioning. The metal layer resistivity may be measured and recorded. The correct positioning of manufacturing operations, including wire bonding positions for example, may be guided by fiducial marks and the resulting features measured relative to such marks.
Another example of a characteristic of a transistor power switch device is its resistance to avalanche breakdown, especially its resistance to large numbers of repetitive avalanche breakdown occurrences. Avalanche breakdown of a transistor power switch can be caused by unclamped inductive switching (‘UIS’). Power transistors such as metal-oxide-silicon field-effect transistors (‘MOSFET’s) inherently have extremely fast switching speeds and consequently are particularly exposed to repetitive avalanche breakdowns. Our co-pending international patent applications PCT/IB2008/055655 and PCT/IB2008/055656 relate to vertical MOSFET power switch devices. The resistance of power switch devices to repetitive high avalanche currents is important, whether or not avalanche diodes are provided to protect the power switch device as in our patent application PCT/IB2008/055655, since the power switch device may be exposed in use to hundreds of millions of avalanche breakdown current occurrences. Measurements are needed of parameters which are symptomatic of product ageing due to avalanche breakdown current occurrences or are likely to contribute to premature avalanche breakdown current failure.